Multidrug resistant (MDR) cancer remains the primary impediment to curative cancer chemotherapy. MDR cancer cells differ from typical tumor cells by dramatically upregulating production of several factors including the drug transporter P-glycoprotein, the cholesterol binding protein caveolin, and components of lipid raft microdomains of cellular plasma membranes. These raft domains are enriched in cholesterol and sphingolipids and play key roles in signal transduction processes. The distinct composition of plasma membranes of MDR cancers may enable selective chemotherapy targeting these cancers. During the last grant cycle, novel small molecules were synthesized that comprise cholesterylamine covalently linked to protein ligands such as biotin. These compounds bind lipid rafts in plasma membranes of cancer cells. Treatment of cancer cell lines with a synthetic biotin-cholesterylamine ligand (ligand #1 ) and the protein Streptavidin (SA) efficiently targets SA to lipid rafts, resulting in rapid clathrin-mediated endocytosis of this protein-ligand complex. This novel system mimics penetration of cells by Cholera toxin, which binds ganglioside GM1 in lipid rafts. This project is based on the hypothesis that ligand #1 will regulate endocytosis of SA linked to endosome-activated toxins daunorubicin and exotoxin in cancer cell lines. Since ligand #1 binds lipid rafts, selective delivery of SA-linked toxins to lipid raft-rich MDR cancer cells will be investigated in vitro and in vivo in murine cancer models. The effectiveness of ligand #1 at enhancing endocytosis of SA- toxins fused to neuropeptide Y, which targets specific receptors on neuroblastoma cells will also be evaluated. This novel approach directed at enhancing endocytosis of surface receptors by targeting to lipid rafts with small molecules could address the major problem in immunotherapy of non-internalized tumor antigens. The hypothesis that ligand-regulated delivery of SA to antigen presenting cells (APCs) will stimulate immune responses will also be tested. Novel immunostimulants will be investigated by fusing SA to the ovalbumin antigen, regulating endocytosis in APCs with ligand #1, and analyzing T-cell activation. This approach could control immunostimulation at the molecular level and yield novel tools for vaccine development. Recruitment of intracellular avidin fusion proteins to plasma membranes by ligand #1 will also be studied in an effort to conditionally regulate cellular growth and death.